Separation of methylamines



9 193., a E. ANDREWS ET AL 9 fi S EPARATI ON OF METHYLAMINES Filed April 18, 1956 .27 FRACTIONATING COLUMNS .1

FRACTIONA COLUMN HEA R {METHYL EMINE DIMETHYL Amms men/ions: C/zesZerEflncZr-ezos dncljeryyluperwe,

. Patented Ali'- 9, 1938 I UNlTED STATES SEPARATION OF METHYLAMINES Chester E. Andrews. Oven-brook, and Le Roy U. Spence, Cheltenham, Po... assignors to Rollin & Hm Company, Philadelphia, Pa,

Application April 18, 1938, Serial No. 75,122

14 Claims.

This invention relates to a process for the separation of monomethylamlne, dimethylamine and trimethylamine from their mixtures, particularly from the mixtures obtained in the catalytic preparation of methylamines.

The product obtained when methanol and ammonia are passed at suitable temperatures over an amination catalyst, after removal of high boiling materials, such as water, methanol and forum [0 aldehyde, consists of a mixture of ammonia,

monomcthylamine, dimethylamine and trimcthylamine. A number of methods for separating this mixture have been suggested. it the mixture is distilled, the ammonia may be removed as W a constant boiling mixture with trimethylamine and if sufllcicnt ammonia is added, the trimethylamine may be completely removed with the ammonia. With the trimethylamine removed, the monomethylamine and dimethylamine may be U separated by distillation. A disadvantage of this method is that it requires large quantities of ammonia and makes necessary a-separate process for recovering it from the constant boiling mixture formed. 35 Another method for the removal of ammonia consists in adding dimethyl ether to the mixed amines and distilling to separate the ammonia as a constant boiling mixture with dimethyl ether. The three methylamines are thus obtained It) tree from ammonia but their separation one from the other is complicated by the formation of constant boiling mixtures of monomethylamine and trimethylamine and by the fact that dimethylamine and trimethylamine boil at very nearly the 5 same temperature. These difliculties have prevented the eificient separation of the methylamines by distillation. An object of this invention is to provide a practical method for the separation of monomethylg amine, dimethylamine and trimethylamine from their mixtures. Another object is to provide a method of separating dimethylamine from trimethylamine. A further object is to separate monomethylamine and trimethylamine from s each other in mixtures containing no dimethylamine. I

In our prior application, Serial No. 538,262 filed May 18, 1931, now Patent No. 2,061,889 granted November 24, 1936, of which this applica- 50 tions is in part a continuation, we have shown that monomethylamine and trimethylamine form a constant boiling mixture which can be utilized to separate trimethylamine from dimethylamine. We have now further found that the composition is or the constant boiling monomethylamine-trimethylamine mixture varies if the distillation is conducted at dlflcrent pressures. We have found that the mixture obtained by distillation at atmospheric pressure contains about 30% by weight of trimethylamine and that as the distilling pressure is increased the trimethylamine concentration decreases until at 60 lbs. gauge it is about 15% and at 210 lbs. gauge about 7%.

In the present invention we utilize our discovery that monomethylamine and trimethylamine form a constant boiling mixture to separate them from dimethylamine, and then by rte-distilling the mono-tr! mixture at different pressures, separate it into its components. Essentially, our process consists in first removing ammonia from the mixed amines, then iractionatlng them to separate trimethylamine and monomethylamine from dimethylamine, and finally by cycling the mixed monoand trlfraction through two fractionating columns, one of which is operated at a relatively high pressure as compared with the other, separate the mono-amine from the tri-.

The mixture of amines used as the starting material may be obtained irom either a catalytic or non-catalytic process of manufacturing methylamines. The ammonia is removed by any suitable method, such as by distilling with dimethyl other. Preferably it is distilled out with part of the trimethylamine and returned to the reaction chamber for further reaction with methanol.

If in the mixture of amines to be separated, the ratio o'l' trimethylamine to monomethylamine exceeds that of the constant boiling mixture, the proportion of the monomethylamine must be increased so ,that all the trimethylaminc can be removed. This may be done by the addition of pure monomethylamine or by the addition of an amine mixture rich in monomethylamine obtained as hereinafter described. If the monomethylamine present exceeds the amount necessary for the constant boiling mixture, the excess may be removed along with the mixture by suitably adjusting the operation of the column.

To further explain our invention, reference will be made to the accompanying drawing which schematically illustrates suitable apparatus for carrying out our process.

In the drawing, letters A, B and C represent fractionating columns that are connected as indicated by pipes I to 1. Letter P represents a pump that forces the distillate from columns A and C into the high pressure column B.

In the operation of this apparatus the mixture of amines prepared by the catalytic amination of alcohol or any other process, after the removal of high boiling materials and ammonia, is conducted through pipe I to i'ractionating column A where it is separated into a residual portion consisting of substantially pure dlmethylamine and a distillate portion containing practically all the monomethylamine and trlmethylamine. Pressure in this column should be maintained as low as expedient, for as explained above, the lower the distilling pressure the greater the concentration of trlmethylamine in the constant boiling mixture evolved. We have found a distilling pressure of 60 pounds gauge to be convenient, for at lower pressures artificial refrigeration must be resorted to to condense the vapors.

The distillate from column A passes through pipes 2 and 3, pump P, and pipe 4 to tractionating column B in which it is again subjected to a fractional distillation, this time under a relatively high pressure. While the specific pressure maintained in column B is not critical, we have found an operating pressure of 210 pounds gauge convenient when the pressure in column A is maintained at 60. pounds. By maintaining a high pressure in column B the composition of the constant boiling monomethylamine-trlmethylamine fraction that is vaporized is altered so that it contains a substantially lower proportion of trimethylamine. The remaining trlmethylamine is drawn oil at the bottom of the column.

The distillate from column B passes through pipe line 5 to column C, and is again fractionated at a low pressure, conveniently 60 pounds gauge. The low pressure in this column again alters the composition of the constant boiling monomethylamine-trlmethylamine mixture, so that the distillate coming of! has substantially the same composition as that coming from column A. The excess monoamine is withdrawn from the bottom of the column. The distillate from column C is drawn off through pipe 6, mixed with the distillate from column A and returned through pipe 3, pump P and pipe 4 to the high pressure distilling column B.

The pipe line 1 connecting the top of fractionating column B and the entrance to column A is provided to permit the recirculation of the mixture high in monomethylamine evolved from column B back to column A. In this way the relative concentration of monomethylamine and trlmethylamine in the initial mixture may be suitably adjusted to insure the presence of sufficient monomethylamine to vaporize in column A all the trlmethylamine as a constant boiling mixture. Where the composition of the initial mixture is such that the addition of more mono is unnecessary, the recirculation of distillate through pipe I is unnecessary. It will be apparent that where the ratio of mono to tri in the initial mixture is very high, the distillate from column A will likewise be high in monomethylamine. Under such operating conditions, it may be desirable to reverse the order in which the mono and tri are removed, by passing the distillate from column A first through column C and then through column B.

The distillation temperatures maintained in the three columns will depend entirely upon the pressure at which they are operated, and while we have indicated a preference for an operating pressure of about 60 pounds gauge in columns A and C and of about 210 pounds gauge in column B, other pressures can be chosen to obtain the same result, so long as the pressure in column B is maintained above that used in columns A and C.

By means of our process a mixture of methylamines containing 59.6% monomethylamine, 28.0% dlmethylamine and 12.4% trlmethylamine, can be separated into 97% pure dlmethylamine, better than 96% pure trlmethylamine and 98% pure monomethylamine. This is accomplished by mixing with the initial amine mixture an equal amount of distillate from column B and maln taining gauge pressures oi 60 lbs. per sq. in. in columns A and C and 210 lbs. per sq. in. in column B. The temperatures in the three columns were 36 C. at the top of column A and 54 C. at the bottom; 76 C. at the top of column B and 103 C. at the bottom; 36 C. at the top of column C and 37 C. at the bottom.

We have described our invention as a complete continuous process for the separation of all three methylamines, but it is apparent that it can be carried out as a batch process and also that the various steps can be utilized as such where it is desired to separate a single or only two of the components. Thus, the steps of adding monomethylamine to the initial mixture and distilling, can be utilized to obtain pure dlmethylamine without subsequently separating the mono from the tri. Also, if dlmethylamine and trlmethylamine are the only desired components, the low pressure distillation in column C can be omitted. The monoamine rich distillate from column B can then, if desired, be reacted with more methanol to convert it to the higher amines. Furthermore, it pure monomethylamine is the desired component, the fractionation in column A can be omitted and both dimethylamine and trlmethylamine separated in column B. It is to be understood that our invention comprises the various steps of our process as well as the complete combination.

It will also be apparent that our process lends itself to various modifications to adapt it to specific operating requirements. Thus, where the mixture of amines to be separated contains noncondensable gases, it is desirable to provide a means for removing these from the system, preferably in the reflux condenser of column A. Or, if the mixture of amines fed to column A contains some high boiling materials, such as methanol and water, these may be removed with the dlmethylamine from the bottom of the column, and later removed by a subsequent distillation. Other similar modifications can be resorted to without departing from the scope of the present invention as defined in the following claims.

We claim:

1. In the process of separating dimethylamine from a mixture of methylamines from which ammonia has been removed the steps of adding to the mixture monomethylamine and distilling to remove the trlmethylamine in a constant boiling monomethylamine-trimethylamine mixture.

2. The process of separating dlmethylamine from its mixtures with trimethylamine from which ammonia has been removed which comprises adding to the mixture sufilcient monomethylamine to remove the trlmethylamine in a constant boiling monomethylamine-trimethylamine mixture and distilling.

3. The process of separating a purified dlmethylamine and a purified trlmethylamine from a mixture of methylamines from which ammonia has been removed which comprises adjusting the monomethylamine concentration of the mixture to permit the removal of trlmethylamine in a constant boiling monomethylamine-trimethylamine mixture, distilling to remove the monoareaeco methylamine and trimethylamine, and redistilling the mixture of monomethylamine and trimethylamine under a higher pressure to separate part of the trimethylamine.

4. The process ofseparating a purified dimethylamine and a purified trimethylamine from a mixture of methylamines from which ammonia has been removed which comprises adjusting the monomethylamine concentration of the mixture to permitthe removal of trimethylamine in a constant boiling monomethylamine-trimethylamine mixture, distilling under about 60 pounds gauge pressure to remove the monomethylamine and trimethylamine, andredistilling the mixture of monomethylamine and trimethylamine under about 210 pounds gauge pressure to separate part of the trimethylamine.

5. The process of separating methylamines containing sufiicient monomethylamine to remove all the trimethylamine in a constant boiling monomethylamine-trimethylamine mixture which comprises separating any ammonia present distilling to remove the monomethylamine and trimethylamine, and redistilling the mixture of monomethylamine and trimethylamine under a higher pressure to separate part of the trimethylamine. l

6. The process ofseparating a mixture of methylamines from which ammonia has been removed which comprises adjusting the monomethylamine concentration of the mixture to permit the removal of trimethylarnine in a constant boiling monomethylamine-trimethirlamine mixture, distilling to remove the monomethylamine and trimethylamine, redistilling the mix-.

ture of monomethylamine and trimethylamine under a higher pressure to remove part of the trimethylamine and redistilling the distillate thus obtained under a pressure not as high to remove a part of the monomethylamine.

- 7. The process of separating a mixture of methylamines from which ammonia has been removed which comprises adjusting the monomethylamine concentration of the mixture to permit theremoval of trimethylamine in a constant boiling monomethylamine-trimthylamine mixture, distilling under about 60 pounds? gauge pressure to remove the monomethylamine and trimethylamine, redistilling the mixture of monomethylamine and trimethylamine under about 210 pounds gauge pressure to remove part of the trimethylamine and redistilling thedistil'iate thus obtained under about 60 pounds gauge pressure to remove a part of the monomethylamine.

8. The process of separating methylamines containing sufficient monomethylamine to remove all the trimethylamine in a-constant boi1 ing monomethylamine-trimethylamine mixture which comprises separating any ammonia present distilling to remove the monomethylamine and trimethylamine, redistilling the mixture of monomethylamine and trimethylamine under a higher pressure to remove part of the trimethylamine and redistilling the distillate thus obtained under a pressure not as high to remove a part of the monomethylamine.

9. The process of separating methylamines containing suflicient monomethylamine to remove all the trimethylamine in a constant boiiing monomethylamine-trimethylamine mixture which comprises separating any ammonia present distilling under about 60 pounds gauge pressure to remove the monomethylamine and trimethylamine, distilling the mixture of monomethylamine and trimethylamine under about 210 pounds gauge pressure to remove part of the trimethylamine and redistilling the distillate thus obtained under about 60 pounds gauge pressure to remove a part of the monomethylamine.

10. The process of separating a mixture con sisting essentially of monomethylamine and trimethylamine which comprises fractionating the mixtureat one pressure and then iractionating the distillate obtained at a diflerent presure, the difference in the fractionating pressures being sufficient to change the composition of the constant boiling monomethylamine-trimethylamine mixture.

11. The process of separating purified monomethylamine from a mixture of methylamines from which ammonia has been removed which comprises fractionating at one pressure toseparate monomethylamine and part of the trimethylamine from dimethylamine, and then fractionating the distillatejat lower pressure to separate part of the monomethylamine.

12. The process of separating amixture consisting,essentiall of monomethylne and trimethylamine which comprises fractionating the mixture under one pressure to separate a part of trimethylamine and then 'fractionating the distillate under lower pressure.

13. The process of separating a mixture consisting essentially o1 monomethylamine and trimethylamine which comprises fractionating the mixture under about 210 pounds gauge pressure to separate a part of trimethylamineancl then fractionating the distillate under about 60 pounds gauge pressure.

14. The processof separating a mixture consisting essentially of monomethylamine and tri-= methylamine which comprises fractionating the mixture underione pressure to separate part of the monoinethylamine, and then fractionating the distillate under higher pressure to separatepartot theitrimethyiamine. V

V CHESTER E. ANDS.

'LE ROY U. SPENCE. 

